This is a quote from the book "On Zero Confirmation Transactions" by Chris Pacia
...versible. That’s not technically true, or at least not true of transactions that have been made recently. In the first number of minutes after a transaction is made, it can theoretically be re
versed (or ‘double spent’) by the sender, assuming he has some technical ability. This means someone could pay you for some merchandise, then steal back the coins afterwards. The more time t
hat passes, however, the more difficult it becomes to reverse a transaction. Typically we say that “unconfirmed” transactions are the easiest to reverse, while the deeper a transaction is in...
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